1. Field of the Invention
The present invention relates to an encapsulation for the connection end or the termination end of an electric strip heater cable which in the core has a plurality of electric conductors, and over the core has an insulating outer sheath made from a thermoplastic fluoropolymer material. The invention also relates to a method for producing such an encapsulation.
2. Discussion of the Related Art
Normally, with the strip beater cables of the type considered two of the conductors are arranged inside and parallel to one another; they are also denoted as bus wires. The latter are embedded, for example, in a plastic material which is constructed to be weakly conducting by means of appropriate additives. If a voltage is applied to the two bus wires (for example a normal AC supply voltage), a loss-affected flow of current arises between the two bus wires through the weakly conducing material, and heat losses are produced. This type of strip heaters is also denoted as self-limiting strip heater cables or as parallel conductor heaters. Instead of being connected via a poorly conducting plastic material, the two bus wires could also be connected via a resistance wire which is wound around them and welded to them at regular intervals.
Strip heater cables of the type considered are usually also further shielded in the manner of a coaxial cable, and therefore also have a conducting layer, for example in the form of a wire mesh, over an intermediate insulation. Only then is the abovementioned outer sheath fitted over this shield.
In order to connect the two bus wires and the shield, these are exposed to a small extent and then connected individually to pigtail leads, for example via crimp conductors. For the purpose of insulation and for protection against external influences, the connecting region is subsequently mostly encapsulated, and this is usually performed simply by bonding. The free ends of the strip heaters are also encapsulated by bonding.
Because of their specific surface properties, the very temperature-resistant thermoplastic fluoropolymers mentioned at the beginning can, however, be bonded tightly and durably only conditionally with an extremely low adhesion using current adhesion methods, In particular, encapsulations produced in this way are very sensitive to mechanical loads.
The encapsulation proposed by the invention comprises a moulded part made from a thermoplastic fluoropolymer material which can be welded to that of the outer sheath of the strip heater cable and is durably welded in a hermetically tight fashion. The encapsulation according to the invention is suitable, in particular, for applications in a rugged environment and the environment which poses stringent safety requirements, such as in aeroplane manufacture.
In the case of strip heaters of the type considered, the outer sheath is usually extruded onto the core containing the conductors. The moulded part used for the encapsulation according to the invention is preferably shaped such that it can also traverse the extruder during the extrusion process of the strip heater outer sheath, and is thereby likewise sheathed by the strip heater outer sheath.
Advantageous, and therefore preferred refinements and improvements of the invention are respectively characterized in the dependent claims.